Varactor tuned cavity



April 28, 1970 G. scHAr-FNER VARACTOR TUNED cAvITY Filed Aug. 9. 1967 y ZOU //fF///// `ta ks e her mi mped United States uPatent p O1 nce ABSTRACT 0F THE DISCLOSURE Varactor tuned cavity having input and output connection loops on opposite sides of the cavity.` A coaxial 5 member extends upwardlyfrom oner end off the cavity and is connected to the other end through a varactor diode coaxially `disposed insidetthe cavity. The` `varactortfdii l. odelis` selected such `that, its` effective" capacitance fat th berlby an RF isolation, niearis extendingperpendicular `to the varactor diode. A capacitance is formed between the coaxial member and the cavity shell at the :onetend `such `that the coaxial member ,isielectrically floatingwvvithir p spect tothe cavity shell. i i t BACKGaoUNuvoa iNvnNrioN: 1i i This invention relates` to improvementsl in highfre-y i quency tunedicavities andparticularly` total tunable cavity which is` tuned `by a., voltage variablecapacitance Idevi` p "anlhasa broadrange otuningt K Tuning circuitsini til` .l

udm television tune "erably an electronicall sele "tllatl` voltage variable p ferred` to asivaractors cavities require tor is` al reversed biase rectifytingtjunction.iniiawseriiic ductori device.` Byl` itseltysuchl` a junctiomhas negligible inductance, the equivalent circuit of the gjunctontathigher frequencies is merely the voltage dependence `junction capacitance, CJ, with a series resistance, RS. When a y package is used to encapsulate the semiconductor Wafer, the external contacts give rise to series lead inductance,

Ls, and the shunting case capacitance, CC. The lead in the form of a strap, wire, Whisker, or spring from the junction to the package contacts contributes substantially to this inductance.`

The majority of tuned circuits utilizeparaHel `resonant 'stribute form;` h 'nherent i I"tuned cavities;

ing range afforded by a varactor diode with a tuned cavity can be severely limited. In distributed component circuits, such as cavities, `the degradation is quite similar to that of lumped component circuits. In design the diode inductance has to be considered. That is, the diode package and lead inductance forms a series resonant circuit With the eiective capacitance of the diode.

It is desirable in constructing high Volume tuned cavities to use as inexpensive a diode as possible. Heretofore ceramic packages, rather expensive, were required to provide a broad tuning range in tuned cavities. It is desired that tuning diodes such as housed in a DO-7 glass package` or a plastic `encapsulated",package be usable with utilising inexpensive `It is a further object of this l*Invention t tunable cavity with a varactor semiconductor device in t vcompsnsated" tunable sovfldoestno ateriallyife uit `operation cavity i coaxial shell to a oatingcoaxial inember` inside the cavity. other feature ,is` the inclusion, of` acapacitance between` `iloating member ableydevicesfA" herygfe utfoutputtinductiv oop iiconductortdewc iasmeansiextendin f,1 the l MBODINIENT 12th1jough output inductive loop, 17 which is preferablyi diameter "las input loop` 11, as bes isposed on tliewsame.`

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Patented Apr. 2s, 1970:`

o provlde a` h1ch-the inherent linductancevof` `thetwdeviceis substanf` ithf` the, hell` fand; extending, `from s, y

d,the` shell `opposite to the Vt'lltage,` y

,Referring `more:particularlytothe drawings likeinum-l i bers indicate like parts` and stnuctural features in the i semiconductive tuning device or diode 19 has a pair of' leads 20 and 21 extending in opposite directions and respectively electrically connected to cover and to member 18 as best seen in FIG. 2. Leads 20 and 21 especially the lead portion inside the encapsulating case of diode 19 provides series inductance 22 (FIG. 3) integrally with diode` 19 as is known.

The bias circuit for reverse biasing the rectifying junction of diode 19 may be provided by a `battery source 23 having its negative terminal grounded, as schematically shown in FIGURE 1, to shell 13. Its positive terminal is connected to member 18 through feedthrough capacitor assembly 24, of known design, and thence through an RF choke 25 which is connected between capacitor 24 and member 118. It is preferred that choke 25 extend perpendicularly with respect to diode 19 and as best seen in FIG- URE 1. The device is completed by a relatively large capacitor, which may be fixed or variablefformed by insulative layer 26 and insulating collar 27 disposed between end portion 14 of shell 13y and member 18 as best seen in FIGURE 2. The effective capacitance of layer 26 and collar 27 is represented in FIGURE 3 as capacitor 28, the one-half wave length line formed by member 18 and shell 13 as represented in FIGURE 3 by block symbol 29. While layer 26 is shown as providing a fixed capacitance, in apractical embodiment a variable capacitance structure of known design can be used.

i An experimental model of the illustrative embodiment was constructed and tested to determine the effective tunf` ing range and designability of such a device. Close agreement between calculated values and performance of the experimental device clearly showed a broad tuning range with the tunable cavity utilizing a lN5139 glass encapsulated diode. Inthe experimental model the tuning range was `arbitrarily selected to be from `470l mI-Iz, to 890 mHZ. (megahertz). By keeping the series tuned capacitance formed by layer 26 and collar 27 in series with the capacitance of varactor diode 19 as large as possible, the voltage variable junction capacitor controlled the resonant frequency of the assembly to` the greatest possible extent. In constructing the embodiment the ratio of the maximum to the minimum capacitance for the tuning range was kept greater than the square of the ratio of the frequencies of the tuning range'. In the example,the ratio of (890 to` 470)2 is equal to 3.58 (this`is\1.892). For the INS 139, the junction capacitance at a reverse bias of 1 volt yielded a junction capacitance of 10.2 pf. (picofarads) while a minimum capacitance at a reverse bias of 60 volts yielded`2.2.pf. The ratio of maximum to minimum capacitance was 4.64.

At the mean frequency of the frequency range the lead length of the varactor was selected such that the inherent inductances were self-resonant with the junction capacitance at itsmean value. In the constructed embodiment, the 1N5139 mean capacitance between the reverse bias of l volt and 60 volts is 4.8 pf. occurring at l0 volts reverse bias. The inductance required to resonate with 4.8 pf. at thetmean frequency of 650 mHz. requires an inductanceof 12.5 nh. (nanohenries). Lead inductance of this diode is.27 nh. per inch and with a 3 nh. internal varactor inductance. Calcualtions showed that required lead length was 0.35 inch.

For a broad tuning range a low characteristic impedance for the transmission line portion of the cavity is desired, i.e. that portion adjacent member 18. A low characteristic impedance includes impedances between and 40 ohms, for example. In the constructed embodiment, the characteristic impedance was selected to be 30 ohms. For acoaxial line (member 1-8 to shell 13) having an outer diameter of 1 inch and an inner diameter of 0.6 inch characteristic impedance turns out to be 30 ohms.

One should also determine the minimum allowable equivalent` transmission line capacity which` is in series with the capacitanceof the varactor. The first step is to determine the cavity resonance at the frequency band limits of, for example 470 and 890 mHz. This step can be done using known expressions for determination of frequency in accordance with series connected capacitance as provided by the varactor 19 and capacitors 26, 27 in series therewith. In the constructed example, the minimum capacitance to be provided by layer 26 and collar 27 was calculated to be 24.5 pf. (Note: This also includes the effective capacitance of the line.)

The electrical length 0 at the mean frequency is determined. In the example, at the mean frequency of 650 mHz. the electrical length of the line without capacity loading` provided by layer26 and collar 27 was determined t to be 0.34 radian or 19.6 degrees. From the electrical length the physical length is determined by the equation:

L=ox/21r (1) the characteristic impedance and the electrical length H be` kept as small as possible which means thatthe `end capac-` ity or the capacity loading should be large.` Further whenH the junction capacitance of diode 19 and the `series lead inductance are made series resonant the mean frequency maximum tuning range yis obtained when the other above lmentioned criteria are met.

From inspection of the equivalent circuit in FIGURE 3 as Well as the constructed illustrated embodiment it is seen that the series inductance 22 of diode 19 will not appear as an insertion lossbetween the input and output connections 10' and 16.

What is claimed is:

1. Microwave structure havingy a cavity formed in a conductive tubular shell with input and output connections in a first end portion of the cavity and having rst and second ends,

the improvement including in combination,

a center conductive member disposed coaxially inside the cavity andhaving thirdk and fourth ends and.

forming a tuned" portion in the cavity,

a capacitor` forming means disposed between `the second end and said member third end,

Va variable Vcapacitance semiconductive `device disposed between said member fourth end and the shell first end and disposed `in said onev end portion and having opposite electrical leads respectively connected to said shell and member, and

RF isolation means electrically coupled to the lead of said semiconductive device which is connected to the center member, said isolation means extending radially from the fourth end of said center member toward and insulated from said shell for providing bias circuit `connections for said device.

2. The structure of claim 1 wherein said device `has leads with an inductance, the effective capacitance of said device being resonant with said inductance at a frequency which is the mean frequency of a band of frequencies in which the structure is to be operated.

3. The structure of claim 2 wherein said cavity has a low characteristic impedance.

4. The structure of claim 2 wherein said input and output connection means each comprise an inductive loop disposed at oppositeends of one diameter inside said.

. shell and located in juxtaposition to said device in said one end portion.

5,. The structure of claim 4 wherein said capacitance forming means comprises a layer of insulative material.

6. The structure of claim 5 wherein said shell has a 5 6 circular cross-section and said device has leads disposed References Cited on the central axis of the cylindrical shell with the device UNITED STATES PATENTS having oppositely extending leads of equal length.

3,108,239 10/1963 Kouerter S33-83 7. The structure of claim 6 wherein said tuned portion has an effective electrical length equal to approximately 5 HERMAN KARL SAALBACH Prima Examiner one-half the wave length of a center frequency in said l ry band of frequemies. L. ALLAHUT, Assistant Examiner 

